Understanding Optics

09-Aug-2011

We’ve decided to put a guide to understanding optics on the website, due to room constraints in the magazine.

Optics reviews by nature cover some fairly technical stuff that requires quite a bit of explanation to anyone reading one for the first time. Rather than repeating this every time we do an optics testfire, we’re now going to put a basic introduction to understanding our optical reviews on here.

Below is a few words about how to get the most out of our testfires of binoculars, scopes etc.
This is largely from the Bushnell bino testfire in Issue 3, for anyone who prefers a hard copy and still has that issue:

Optically Speaking

This is the crux of the matter, what can you see!

The Resolution score is the most important feature when testing binoculars and really tells you something about the quality of their optical components. It shows up aberrations such as spherical, coma and astigmatism, provides some measure of contrast, and allows you to do comparative tests against a recognised test pattern that actually mean something. Reviewers making statements like ‘clear and sharp’ tell you nothing other than that person’s purely subjective opinion.

The second most important is the Flatness of Field test which tells you how focused the image remains as you move out towards the edge of the field of view. Curvature of field and spherical aberration are the worst culprits here. Binos with a flat field are nicest to use, but as you do your evaluating in the centre, resolution here is still the most important. Binoculars that don’t remain in focus over at least 2/3’s of their field are not a pleasant viewing experience.

Poor alignment between both barrels renders binoculars useless, as they produce images in different places that your eyes and brain strain to realign. Even a slight misalignment will give you a headache. Poor lens alignment (vignetting) within the barrel affects field of view and light transmission.

All high magnification optics show some distortion (image not reproduced in its actual size and shape), and in fact a small amount of pincushion distortion is introduced by most binocular manufacturers to minimalise another form called rolling distortion. This is when you pan along something like a fence line some distance from you and at right angles (perpendicular) to you, and the posts increase in size as they move into the centre of your view then decrease again as they go out. This is because those posts in the centre of the field of view are closer to you than the posts out to the left and right, and therefore look larger. Pin cushion distortion enlarges objects towards the edges and can negate rolling distortion if balanced correctly.

Accurate colour rendition isn’t particularly important in hunting optics for viewing (eg binos, spotters), but more so for recording optics like cameras. Colour bleeding or flare is important as it affects resolution when the colours aren’t distinct.

Twilight performance is important for some, depending on your style of hunting. Once again it is what you can resolve in low light that matters, not whether the image is ‘bright’ as you’ll see some reviewer’s state. A bright blurry image is of little use to anyone.

The fluoride lenses used in the various HD or APO top of the line models do provide increased contrast and a richer image. This contrast increase does lead to better performance in the resolution test, and an HD model is generally capable of a good half level better on the test pattern verses the equivalent standard lens model.

We will be updating this optical discussion from time to time as we become aware of further things that need explaining.